2014-07-29 08:39:59 +08:00
# Snappy
Snappy is a light-weight layout framework which wraps AutoLayout with a nicer syntax. Snappy has its own layout DSL which provides a chainable way of describing your NSLayoutConstraints which results in layout code that is more concise and readable.
Snappy supports iOS and Mac OS X.
2014-07-29 20:02:27 +08:00
> Snappy uses some Swift only features like function overloading it cannot be used from Objective-C. Because of this we’ ’
2014-07-29 08:39:59 +08:00
## What's wrong with NSLayoutConstraints?
Under the hood Auto Layout is a powerful and flexible way of organising and laying out your views. However creating constraints from code is verbose and not very descriptive.
Imagine a simple example in which you want to have a view fill its superview but inset by 10 pixels on every side
```swift
let superview = self;
let view1 = UIView()
view1.setTranslatesAutoresizingMaskIntoConstraints(false)
view1.backgroundColor = UIColor.greenColor()
superview.addSubview(view1)
let padding = UIEdgeInsetsMake(10, 10, 10, 10)
superview.addConstraints([
NSLayoutConstraint(
item: view1,
attribute: NSLayoutAttribute.Top,
relatedBy: NSLayoutRelation.Equal,
toItem: superview,
attribute: NSLayoutAttribute.Top,
multiplier: 1.0,
constant: padding.top
),
NSLayoutConstraint(
item: view1,
attribute: NSLayoutAttribute.Left,
relatedBy: NSLayoutRelation.Equal,
toItem: superview,
attribute: NSLayoutAttribute.Left,
multiplier: 1.0,
constant: padding.left
),
NSLayoutConstraint(
item: view1,
attribute: NSLayoutAttribute.Bottom,
relatedBy: NSLayoutRelation.Equal,
toItem: superview,
attribute: NSLayoutAttribute.Bottom,
multiplier: 1.0,
constant: -padding.bottom
),
NSLayoutConstraint(
item: view1,
attribute: NSLayoutAttribute.Right,
relatedBy: NSLayoutRelation.Equal,
toItem: superview,
attribute: NSLayoutAttribute.Right,
multiplier: 1.0,
constant: -padding.right
)
])
```
Even with such a simple example the code needed is quite verbose and quickly becomes unreadable when you have more than 2 or 3 views.
Another option is to use Visual Format Language (VFL), which is a bit less long winded.
However the ASCII type syntax has its own pitfalls and its also a bit harder to animate as `NSLayoutConstraint.constraintsWithVisualFormat` returns an array.
## Prepare to meet your Maker!
Heres the same constraints created using ConstraintMaker
```swift
let padding = UIEdgeInsetsMake(10, 10, 10, 10)
view1.snp_makeConstraints { make in
make.top.equalTo(superview.snp_top).with.offset(padding.top) // with is an optional semantic filler
make.left.equalTo(superview.snp_left).with.offset(padding.left)
make.bottom.equalTo(superview.snp_bottom).with.offset(-padding.bottom)
make.right.equalTo(superview.snp_right).with.offset(-padding.right)
}
```
Or even shorter
```swift
view1.snp_makeConstraints { make in
make.edges.equalTo(superview).with.insets(padding)
return // this return is a fix for implicit returns in Swift and is only required for single line constraints
}
```
Also note in the first example we had to add the constraints to the superview `superview.addConstraints` .
Snappy however will automagically add constraints to the appropriate view.
Snappy will also call `view1.setTranslatesAutoresizingMaskIntoConstraints(false)` for you.
## Not all things are created equal
> `.equalTo` equivalent to **NSLayoutRelation.Equal**
> `.lessThanOrEqualTo` equivalent to **NSLayoutRelation.LessThanOrEqual**
> `.greaterThanOrEqualTo` equivalent to **NSLayoutRelation.GreaterThanOrEqual**
These three equality constraints accept one argument which can be any of the following:
#### 1. ViewAttribute
```swift
make.centerX.lessThanOrEqualTo(view2.snp_left)
```
ViewAttribute | NSLayoutAttribute
------------------------- | --------------------------
view.snp_left | NSLayoutAttribute.Left
view.snp_right | NSLayoutAttribute.Right
view.snp_top | NSLayoutAttribute.Top
view.snp_bottom | NSLayoutAttribute.Bottom
view.snp_leading | NSLayoutAttribute.Leading
view.snp_trailing | NSLayoutAttribute.Trailing
view.snp_width | NSLayoutAttribute.Width
view.snp_height | NSLayoutAttribute.Height
view.snp_centerX | NSLayoutAttribute.CenterX
view.snp_centerY | NSLayoutAttribute.CenterY
view.snp_baseline | NSLayoutAttribute.Baseline
#### 2. UIView/NSView
if you want view.left to be greater than or equal to label.left :
```swift
// these two constraints are exactly the same
make.left.greaterThanOrEqualTo(label)
make.left.greaterThanOrEqualTo(label.snp_left)
```
#### 3. Strict Checks
Auto Layout allows width and height to be set to constant values.
if you want to set view to have a minimum and maximum width you could pass a primitive to the equality blocks:
```swift
// width >= 200 & & width < = 400
make.width.greaterThanOrEqualTo(200)
make.width.lessThanOrEqualTo(400)
```
However Auto Layout does not allow alignment attributes such as left, right, centerY etc to be set to constant values.
So if you pass a primitive for these attributes Snappy will turn these into constraints relative to the view’ s superview ie:
```swift
// creates view.left = view.superview.left + 10
make.left.lessThanOrEqualTo(10)
```
You can also use other primitives and structs to build your constraints, like so:
```swift
make.top.snp_equalTo(42)
make.height.snp_equalTo(20)
make.size.snp_equalTo(CGSizeMake(50, 100))
make.edges.snp_equalTo(UIEdgeInsetsMake(10, 0, 10, 0))
make.left.snp_equalTo(view).offset(UIEdgeInsetsMake(10, 0, 10, 0))
```
## Learn to prioritize
> `.prority` allows you to specify an exact priority
> `.priorityHigh` equivalent to **UILayoutPriority.DefaultHigh**
> `.priorityMedium` is half way between high and low
> `.priorityLow` equivalent to **UILayoutPriority.DefaultLow**
Priorities are can be tacked on to the end of a constraint chain like so:
```swift
make.left.greaterThanOrEqualTo(label.snp_left).with.priorityLow();
make.top.equalTo(label.snp_top).with.priority(600);
```
## Composition, composition, composition
Snappy also gives you a few convenience methods which create multiple constraints at the same time.
#### edges
```swift
// make top, left, bottom, right equal view2
make.edges.equalTo(view2);
// make top = superview.top + 5, left = superview.left + 10,
// bottom = superview.bottom - 15, right = superview.right - 20
make.edges.equalTo(superview).insets(UIEdgeInsetsMake(5, 10, 15, 20))
```
#### size
```swift
// make width and height greater than or equal to titleLabel
make.size.greaterThanOrEqualTo(titleLabel)
// make width = superview.width + 100, height = superview.height - 50
make.size.equalTo(superview).offset(CGSizeMake(100, -50))
```
#### center
```swift
// make centerX and centerY = button1
make.center.equalTo(button1)
// make centerX = superview.centerX - 5, centerY = superview.centerY + 10
make.center.equalTo(superview).offset(CGPointMake(-5, 10))
```
You can chain view attributes for increased readability:
```swift
// All edges but the top should equal those of the superview
make.left.right.and.bottom.equalTo(superview)
make.top.equalTo(otherView)
```
## Hold on for dear life
Sometimes you need modify existing constraints in order to animate or remove/replace constraints.
In Snappy there are a few different approaches to updating constraints.
#### 1. References
You can hold on to a reference of a particular constraint by assigning the result of a constraint make expression to a local variable or a class property.
You could also reference multiple constraints by storing them away in an array.
```swift
var topConstraint: Constraint? = nil
...
// when making constraints
view1.snp_makeConstraints { make in
self.topConstraint = make.top.equalTo(superview).with.offset(padding.top)
make.left.equalTo(superview).with.offset(padding.left)
}
...
// then later you can call
self.topConstraint.uninstall()
```
### 2. snp_remakeConstraints
`snp_remakeConstraints` is similar to `snp_makeConstraints` , but will first remove all existing constraints installed by Snappy.
```swift
func changeButtonPosition() {
self.button.snp_remakeConstraints { make in
make.size.equalTo(self.buttonSize)
if topLeft {
make.top.left.equalTo(10)
} else {
make.bottom.equalTo(self.view).offset(-10)
make.right.equalTo(self.view).offset(-10)
}
}
}
```
## TODO
* Eye candy
* Example projects
* Tests
